Fluorescent lighting has been around for many years now. This form of lighting started out as a highly efficient alternative for incandescent light bulbs, but has recently been surpassed by LED lighting in terms of efficiency and power consumption, and also in other aspects as set out below.
Fluorescent lamps generally comprise a tube filled with an inert gas and a small amount of mercury, capped at both ends with double pinned end caps. The end caps contain a glow wire to preheat the gasses inside the tube and to vaporize the mercury in order to assist with ignition of the fluorescent lamp. After the user turns on a main switch (e.g. a wall switch or a cord switch on the ceiling), the fluorescent lamp is ignited, and heat generated by the conducted current keeps the fluorescent lamp in operational condition. To facilitate these starting conditions and to limit current through the fluorescent lamp during operation, and thus limit the power consumed, a ballast is usually fitted in the fluorescent luminaire, connected between the mains power supply and the fluorescent lamp, and power is supplied to the lamp via the ballast.
When first introduced, the only available ballasts were simple inductive or reactive elements placed in series with the power supply to the fluorescent lamp, which limit consumed power by limiting the AC current as a result of the frequency dependent impedance of the inductor. An undesirable result is a relatively low power factor and relatively high reactive power. These types of ballasts are usually referred to as magnetic ballasts.
More recently other types of ballasts have been introduced, such as electronic ballasts. These ballasts usually first convert AC mains power into DC power, and subsequently convert the DC power into high frequency AC power to drive the fluorescent lamp.
Electronic ballasts can further be categorized into two types: constant current ballasts and constant power ballasts. Most electronic ballasts are constant current ballasts, designed to deliver current at a substantially constant amplitude. These ballasts can be modelled as a constant AC current source. These ballasts typically comprise a self-protection/self-correcting mechanism to avoid potential problems of maintaining a constant current. A constant power ballast is designed to deliver substantially constant power and the output current will vary depending on the load to attempt to maintain the design power output. If the operating power is below the design output level, constant power ballasts usually try to increase the output current to come closer to the design power level.
LED lamps are more efficient than fluorescent lamps, and have many other advantages. For example, no mercury is required for LED lamps, the light output from LED lamps is more directional, power can be more easily control or regulated, and the lifetime of LEDs is generally much longer than fluorescent lamps. Thus, replacing fluorescent lamps with LED lamps is often desirable, and it is also desirable to be able to fit replacement LED lamps into existing luminaires designed for fluorescent lamps without needing to modify the luminaire. However, an LED lamp typically operates differently when used with different types of ballasts. In some cases, a straightforward replacement of a florescent lamp by an LED lamp in a fluorescent luminaire results in a failure of the entire luminaire.
An LED lamp arrangement compatible with all three types of ballasts mentioned above (magnetic ballasts, constant current ballast, and constant power ballast) is described in the applicant's U.S. Pat. No. 9,832,837, herewith incorporated by reference in its entirety. This lamp arrangement comprises a plurality of LEDs arranged in a plurality of groups switchable between serial connection and parallel connection, depending on whether the ballast is a magnetic ballast or an electronic ballast, and further comprises an inductive element and a switch which can be closed to short the inductive element, depending on whether the electronic ballast is a constant power ballast or a constant current ballast.
Recently, there is an increasing demand for a universal LED lamp which can be fitted into fluorescent luminaires regardless of the type of ballast in the luminaire, and also luminaires without any ballast. Such a luminaire may for example be a luminaire which was originally designed for a fluorescent lamp but its ballast has been removed. This can happen for example when the ballast is too old and/or is damaged after years of operations. In such cases, many users want to get rid of the ballast but still want to keep the luminaire, or install a new luminaire without any ballast. Such a universal LED lamp can be installed in a luminaire without needing to determine whether there is a ballast in the luminaire or what type of the ballast is in the luminaire, enabling the manufacture and stocking of a single design of LED lamp suitable for use in any fluorescent luminaire.